1. Field of the Invention
This invention relates to dehydrogenative polymerization of silanes to polysilanes, particularly by employing catalysts of transition-metal silyl derivatives therefor.
2. The Prior Art
Recently polysilane polymers -(SiRR')x-, have attracted a great deal of attention because of the development of various technological applications. Some polysilanes, on thermal treatment, yield polycarbosilanes that can be spun into fibers and converted to SiC fibers having high tensile strength and oxidation resistance. Such SiC fibers are capable of reinforcing ceramic materials. Polysilanes also possess remarkable electronicpproperties that allow them to be used as dopable semiconductors, photoinitiators in olefin polymerization, photoresists, photoconductors, and non-linear optical materials.
These applications have been made possible by the development of polysilane syntheses based on the Wurtz-type coupling reaction of appropriate dichlorosilanes with an alkali metal in an inert solvent. This synthetic method is the only useful route to polysilanes and has served well to allow the above applications to evolve. Use of the Wurtz-type coupling method suffers from a number of inherent difficulties including poor control of polymer molecular weight, the production of large quantities of NaCl side product, and the dangers of handling large quantities of molten alkali metals. Wider application of polysilane polymers was therefore hampered by the need for new polymerization reactions.
Then Harrod and others reported that titanocene and zirconocene alkyl derivatives are active catalysts for the dehydrogenative coupling of primary silanes RSiH.sub.3 to linear polysilanes with ca. 10-20 Si atoms eg: ##STR1## wherein as used above and below, Me is methyl, R is Phenyl, Alkyl, M' is Ti, Zr and Cp is .eta..sup.5 -C.sub.5 H.sub.5. See Woo, H. G.; Tilley, T. D., J. Am. Chem. Soc. 1989, 111, 8043-44, particularly footnote 2, incorporated herein by reference.
The above catalysts are referred to herein as the M-C bonded species (metal-carbon) while the invention, described below, is referred to as the M-Si bonded species (metal-silicon).
However in the Harrod systems, above, polymerization times are relatively slow, requiring up to several days for the alkyl-based (M-C) catalyst and the degrees of polymerization of such polysilanes are limited to about 20.
Accordingly there is a need and market for an improved process of catalytic polymerization of silanes that overcomes the above prior art shortcomings.
There has now been discovered a method for the improved polymerization of silanes to polysilanes that employs catalysts of M-Si bonded species in which the polymerization reaction is carried out within minutes rather than days and which produces polysilanes with degrees of polymerization up to 70-85 or more.